The separation and collection of particles/aerosols from an airstream or other fluid streams are of concern in two contexts: first, for purposes of analyzing the type and concentration of such particles/aerosols entrained in the fluid and, second, for clearing particles/aerosols from the fluid stream. Additionally, it is sometimes important to classify particles entrained in a fluid stream by size. For example, this technology may be employed in the detection of airborne biological or chemical warfare agents, the detection of biological contamination in confined spaces, such as aircraft or hospitals, or the detection of industrial pollutants (either in ambient fluid or in the effluent of smokestacks).
Much effort has been expended in the past in the detection and classification of particles or aerosols in fluid streams. Impactors have been used for collecting aerosol particles for many decades. In the earliest embodiments, a stream of fluid containing the particles was accelerated toward an impactor plate. Due to their inertia, the particles hit the impactor plate and were collected there while the fluid was deflected to the side. With these types of impactors, only heavy particles were collected while particles below a certain "cut size" were carried away by the fluid stream.
However, a significant cause of inaccuracy in such impactors results from the deposition of particles on surfaces of the impactor other than the intended collection surfaces. This phenomenon reduces the accuracy of measurement of total particle mass concentration and of the size-fractionation of particles, since such losses cannot be accurately estimated for aerosols having varying size, shape, or chemistry. Additionally, particles may either reentrain in the fluid stream or bounce from the impactor's collection surface upon impact. To remedy this problem, "virtual" impactors have been developed that separate particles from a fluid stream by forces other than impaction. Virtual impactors may operate on a number of different principles, but all avoid actual "impact" as a means to separate particles from a fluid in which the particles are entrained and rely on differences in particle mass to induce inertial separation. Specifically, a particle-laden fluid stream is directed toward a surface presenting an obstruction to the forward movement of the fluid stream. The surface includes a void at the point where the particles would normally impact the surface. When a major portion of the fluid stream changes direction to avoid the obstruction presented by the surface, fine particles remain entrained in the deflected major portion of the fluid stream. Heavier or more dense particles, on the other hand, fail to change direction and are collected in a region of relatively stagnant fluid (a "dead air zone") that is created near the surface. The heavier particles entrained in a minor portion of the fluid stream enter the void defined through the surface, where they can be captured or analyzed.
Some examples of virtual impactors can be found in U.S. Pat. Nos. 3,901,798; 4,670,135; 4,767,524; 5,425,802; and 5,533,406. Because typical virtual impactors do not actually collect particles themselves, but merely redirect them into two different fluid streams according to their mass, they are essentially free of the problems of particle bounce and reentrainment associated with actual impactor devices. Still, particle "wall loss," i.e., unintended deposition of particles on various surfaces of virtual impactor structures, especially at curved or bent portions, remains a challenge with many virtual impactors because typically many stages or layers of virtual impactors are required to complete particle separation.
Therefore, a need exists for a virtual impactor that separates particles from a fluid stream more efficiently and without substantial particle wall loss.